Many medical diagnoses depend upon measurement of the concentration of the constituents of plasma. One method of measuring these concentrations is to withdraw blood from the patient and then analyze the blood. The patent to Coggeshall, U.S. Pat. No. 3,785,772, for example, discloses a device having a pair of syringes to withdraw blood from a patient, a dialysis membrane to separate a particular blood constituent from the blood, a reactant which reacts with the chosen blood constituent to form a reactant-blood constituent complex, the concentration of which is proportional to the concentration of the blood constituent, and a detector to determine the concentration of the reactant-blood constituent complex. Coggeshall, however, must replace the reactant after each measurement because the reactant and the blood constituent form an irreversible complex, and the Coggeshall system cannot give measurement of the instantaneous change in the concentration of the blood constituent in the blood, because much time is taken to remove the blood from the body and obtain a reaction with the reactant.
Merrill, U.S. Pat. No. 3,638,639, also shows measurement of blood constituents outside the body. In the Merrill patent, a catheter is inserted into the blood and lipids are passed through a membrane in the catheter and are dissolved in a solvent which is sent out of the body to be analyzed.
The Kronick et al U.S. Pat. No. 3,939,350 shows a system for carrying out immunoassays using fluorescense to indicate the presence of a ligand to be detected. An analog liquid is bound to a transparent sheet and contacted with aqueous assay solution containing the ligand to be detected associated with fluorescent molecules. The ligands become bonded to the sheet and light is passed therethrough to cause fluorescense.
Brumley, U.S. Pat. No. 3,123,066, Polyanya, U.S. Pat. No. 3,461,856, and Rybak, U.S. Pat. No. 3,787,119, all disclose means to measure properties of the blood in vivo. They comprise optical catheters inserted into the blood for measuring the intensity of light reflected from the blood, thereby indicating the blood's oxygen content. In addition, Rybak can measure blood flow and blood pressure. None of the above-mentioned patents are specifically designed, however, for measuring the concentration of low molecular weight plasma constituents, such as glucose.
Some probes have been devised to measure low molecular plasma constituents, but these are based upon electrochemical reaction. The response of these devices is related to the diffusion rate across the membrane of the probe. Since the membranes used to separate the active electrode from the plasma become fouled after a period of time, the calibration characteristics of the device change with time, making the device inaccurate.